Contact adhesives are a special type of adhesive in which the two parts to be joined are bonded with application of pressure after the adhesive has been applied to the substrate on both sides and after a corresponding open waiting time. In a similar manner to pressure-sensitive adhesives, contact adhesives are used exclusively in the dry state after a film has formed, in particular for bonding non-absorbent substrates or if the use of machines must be avoided for the production of the adhesive bonds. Contact adhesives essentially differ from pressure-sensitive adhesives in that polymer films based on pressure-sensitive adhesives have the ability, due to their viscoelastic properties, to wet a very wide variety of substrates, even at very low pressure, and to adhere to them, but at lower adhesive strength. In the case of contact adhesives, diffusion of the two polymer films into one another only commences when the polymer films of two substrates coated with contact adhesives are brought into contact with one another for a few seconds with application of pressure, and a uniform polymer film is formed. The adhesive bonds produced using pressure-sensitive adhesives therefore have significantly lower adhesive strength (they only adhere), in particular immediately after formation of the bond and at elevated temperature, than adhesive bonds based on contact adhesives. In addition, adhesive bonds based on pressure-sensitive adhesives, in contrast to adhesive bonds produced using contact adhesives, do not withstand static long-term peel and/or long-term shear loads. Adhesive bonds produced using contact adhesives develop high adhesive strength immediately after production of the bond.
Contact adhesives are normally prepared on the basis of polychloroprene, which is dissolved in organic solvents. However, these adhesives are disadvantageous because they contain toxic and flammable solvents (R. S. Whitehouse, Crit. Rep. Appl. Chem., No. 16, pg. 2 (1987)). Attempts have therefore been made to prepare water-based contact adhesives.
The disadvantages of aqueous dispersions of this type are their low adhesive strength in dispersions prepared on the basis of polychloroprene, the "cold flow" in the case of polyacrylate dispersions (in particular in the region of critical adhesive bonds, for example edges), and the low adhesive strength or low adhesion in the case of dispersions based on ethylene-vinyl acetate (E/VAc) (DE-3737 630 A1 (U.S. Pat. No. 4,975,481)).
The use of dispersions based on E/VAc in contact adhesive processes can be improved by the use of plasticizers (EP 321 868 A2). The use of such dispersions is also improved if formulated with resins which act as tackifiers and if small amounts of organic solvents are added (EP 315 070 A2, U.S. Pat. No. 4,975,481). Another process describes the foaming of the applied adhesive (based on ethylene-vinyl acetate) before the two parts to be joined are bonded (U.S. Pat. No. 4,960,802). The use of a mixture of two dispersions of low and high glass transition temperature is described in DE 39 20 935 A1. The use of such dispersions as contact adhesives results in good peel strengths, but there is no mention of the static load-bearing capacity of the adhesive bonds. The preparation of a contact adhesive whose copolymer is distinguished by a very broad molecular weight distribution is described in U.S. Pat. No. 4,477,622.
DE 29 15 887 A1 (U.S. Pat. No. 4,322,516), DE 34 46 565 A1 (U.S. Pat. No. 4,831,077) and DE 35 31 601 A1 (U.S. Pat. No. 4,997,879) describe the preparation of dispersions based on ethylene-vinyl acetate or ethylene-vinyl acetate-acrylate which ar distinguished by particularly good adhesion and increased heat resistance. However, dispersions of this type are not suitable for use as contact adhesives unless formulated with further compounds, and have, in particular, no resistance to static long-term loading by shear forces or peeling.
The object was, therefore, to find an aqueous contact adhesive which gives peel-resistant and/or shear resistant adhesive bonds and has good adhesion to various substrates and high cohesion at elevated temperatures, even without further formulation.